Authorities in southern Somalia are searching for four men working on a polio vaccination campaign who were kidnapped by suspected al-Shabab militants.

The two aid workers and two drivers, all Somali nationals, were abducted Sunday near the town of Luuq in the Gedo region. They were believed to be working on a program funded by the World Health Organization but hired by local authorities.

"The two aid workers were busy mobilizing the people and preparing children for vaccines," said Mohamed Hussein Ganey, the chairman of Luuq.

The men were well known in the city. Authorities say they are working to make sure the men are returned unharmed.

"We are trying to free them, but we don't know a place to go. They are taking them to a new direction," Ganey told VOA's Somali service Tuesday. "This is unfortunate, and other aid workers are shocked. There is insecurity and risk for everyone."

The WHO's Somalia branch said it heard reports of an incident near Luuq, but did not believe it involved WHO staff members.

"We received reports of a security incident involving one or more members of the local community hired by local authorities for polio vaccination campaigns. According to our information, no WHO staff was involved in the incident," WHO Somalia said in a written statement.

Colonel Deeq Abdi Khaliif, commander of the Somali army in Luuq, says the workers were in their homes Sunday when gunmen broke in, took them by force and drove them to a remote area.

"At 4 a.m. local time, Maganey village, close to Luuq, was attacked by gunmen from al-Shabab, and [they] kidnapped the ... men who were contractors for WHO. We are now pursuing them to free the abducted men," Khaliif told VOA's Somali service.

Before the kidnapping, the aid workers had received death threats indicating that al-Shabab objected to what they were doing. The al-Qaida-linked extremist group has not allowed polio vaccination programs in districts it controls.

The group has also spread false rumors that the vaccines contain the AIDS virus or cause sterility. The campaign has led some parents to refuse to have their children vaccinated.

Despite efforts by international organizations to eradicate the disease, polio outbreaks persist in Somalia. A 2015 WHO report said that 420,000 children living in parts of Somalia not under government control had not been vaccinated.